Patent Document 1 discloses one example of a vehicle wheel rim having a non-constant thickness from a plate material having a constant thickness. In the manufacturing method of the wheel rim having a non-constant thickness of Patent Document 1, a cylindrical hollow material having a constant thickness is manufactured from a flat plate material having a constant thickness, and then the cylindrical material is formed to a cylindrical hollow member having a non-constant thickness by flow-forming such as a flow-turning, spinning, etc. The cylindrical member is roll-formed to a wheel rim configuration so that the vehicle wheel rim having a non-constant thickness is manufactured.
However, there are the following problems with the manufacturing method of the vehicle wheel rim having a non-constant thickness using flow-forming:    (i) The equipment used in the flow-forming may be expensive.    In flow-forming, since a roll for pressing the tubular material to a mandrel may be moved in two directions, an axial direction of the material and a thickness direction of the material, the flow-forming equipment may be expensive multiple times as compared with an ironing apparatus where a punch may be moved in only one direction.    (ii) Productivity of the flow-forming may be low.    The productivity of flow-forming may be about one third of that of forming using an ironing apparatus. If a rim manufacturing line is diverged to three subsidiary lines and each of the three subsidiary lines is provided with flow-forming equipment, the problem of productivity will be solved. However, since three sets of flow-forming equipment must be provided, the equipment cost and the amount of space for equipment placement are three times of those of a single set of flow-forming equipment.    (iii) Pushing flaws made by the flow-forming roll may remain at a surface of the material, and the appearance quality decreases.